Method and system for characterizing a port fuel injector

Abstract

Methods and systems are provided for calibrating engine port injectors. After pressurizing a low pressure fuel rail, a lift pump may be disabled and port injector variability may be correlated with a measured fuel rail pressure drop at each port injection event by sweeping injection pressure while maintaining injection voltage, and then sweeping injection voltage while maintaining injection pressure. A port injector variability map learned as a function of injection voltage and injection pressure is then transformed into a map learned as a function of injection current and injection pressure by accounting for injector variability caused due to changes in injector temperature.

The invention claimed is: 
     
       1. A method for an engine, comprising:
 port fueling an engine with a fuel rail pressure above a threshold pressure, a lift pump disabled, and while maintaining a base injection pressure and varying injection voltage and maintaining a base injection voltage and varying injection pressure; 
 learning variability between port injectors of the engine based on a measured drop in the fuel rail pressure, as a function of each of injection pressure and injection voltage, for each injection event of the port fueling; and 
 adjusting subsequent port fueling of the engine based on the learning. 
 
     
     
       2. The method of  claim 1 , further comprising temporarily operating the lift pump to raise the fuel rail pressure above the threshold pressure, and then disabling the lift pump. 
     
     
       3. The method of  claim 2 , wherein the threshold pressure is determined based on a fuel line pressure of a fuel line coupling the lift pump to a port injection fuel rail, and wherein the fuel rail pressure is maintained above the fuel line pressure after disabling the lift pump via a pressure relief valve coupled to the fuel line at an inlet of the port injection fuel rail. 
     
     
       4. The method of  claim 1 , further comprising, responsive to the fuel rail pressure dropping below the threshold pressure during the learning, temporarily suspending the learning, operating the lift pump to raise the fuel rail pressure above the threshold pressure, then disabling the lift pump and resuming the learning. 
     
     
       5. The method of  claim 1 , wherein the learning the variability between port injectors of the engine includes, for each port injector, updating each of an injector offset and a slope of a function correlating injected fuel mass to injector pulse-width. 
     
     
       6. The method of  claim 5 , wherein a fuel pulse-width commanded during the port fueling is based on engine speed, and wherein the learning is further based on the commanded fuel pulse-width, the learned variability attributed to the injector offset when the commanded fuel pulse-width is lower than a threshold pulse-width, the learned variability attributed to the injector slope when the commanded fuel pulse-width is higher than the threshold pulse-width. 
     
     
       7. The method of  claim 5 , wherein the adjusting subsequent port fueling of the engine based on the learning includes commanding a fuel pulse-width to a given port injector of the engine based on the updated injector offset and the updated slope for the given port injector. 
     
     
       8. The method of  claim 5 , wherein the adjusting further includes:
 for a given port injector;
 estimating an injector current as a function of the injection voltage and a measured fuel rail temperature; 
 transforming the learned variability, including each of the updated injector offset and the updated slope, as the function of the injection voltage to an updated variability as a function of the estimated injector current; and 
 commanding a fuel pulse-width to the given port injector based on the updated variability. 
 
 
     
     
       9. The method of  claim 1 , wherein the learning the variability as a function of each of injection pressure and injection voltage includes, while maintaining injection voltage at the base injection voltage, learning the variability as a correlation between the measured drop in fuel rail pressure and the varying injection pressure. 
     
     
       10. The method of  claim 9 , wherein the learning the variability as the function of each of injection pressure and injection voltage further includes, while maintaining injection pressure at the base injection pressure, learning the variability as a correlation between the measured drop in fuel rail pressure and the base injection voltage, and a correlation between the measured drop in fuel rail pressure and a higher than base injection voltage. 
     
     
       11. The method of  claim 1 , wherein the port fueling with the lift pump disabled and the learning are performed after an engine temperature is above a threshold temperature, the method further comprising, when the engine temperature is below the threshold temperature, delaying the port fueling with the lift pump disabled and the learning. 
     
     
       12. The method of  claim 1 , wherein the port fueling includes a predetermined number of fuel injection events, and wherein, during the port fueling, each of the port injectors of the engine is operated sequentially. 
     
     
       13. A method for an engine, comprising:
 operating a lift pump to raise a port injection fuel rail pressure above a threshold pressure and then disabling the lift pump; 
 for a predefined number of subsequent port injection events:
 sequentially operating a plurality of port injectors of the engine while maintaining a base injection pressure and sweeping injection voltage and while maintaining a base injection voltage and sweeping injection pressure; 
 correlating a fuel rail pressure drop at each port injection event, as a function of injection pressure and injection voltage, to a parameter indicative of injector variability for a corresponding port injector; and 
 after the predefined number of subsequent port injection events, adjusting a fuel pulse-width commanded to each port injector based on the parameter indicative of injector variability for the corresponding port injector. 
 
 
     
     
       14. The method of  claim 13 , wherein the correlating includes:
 correlating the fuel rail pressure drop at each port injection event to the parameter indicative of injector variability as a function of injection pressure by sweeping injection pressure while maintaining injection voltage at a first setting; and 
 then correlating the fuel rail pressure drop at each port injection event to the parameter indicative of injector variability as a function of injection voltage by maintaining injection pressure while transitioning injection voltage between the first setting and a second setting, higher than the first setting. 
 
     
     
       15. The method of  claim 14 , wherein sequentially operating each port injector of the engine includes commanding a pulse-width at each port injection event based on engine speed, wherein the parameter indicative of injector variability includes, for each port injector, one or more of an offset and a slope of a function correlating injected fuel mass to injector pulse-width, and wherein the correlating further includes correlating the fuel rail pressure drop to the offset when the engine speed is lower than a threshold speed, and correlating the fuel rail pressure drop to the slope when the engine speed is higher than the threshold speed. 
     
     
       16. The method of  claim 13 , wherein the threshold pressure is a first threshold pressure, the method further comprising, before disabling the lift pump, operating a high pressure fuel pump coupled downstream of the lift pump to raise a direct injection fuel rail pressure above a second threshold pressure, higher than the first threshold pressure. 
     
     
       17. The method of  claim 13 , wherein the predefined number of subsequent port injection events is adjusted to enable each port injector of the engine to be sequentially operated at least a threshold number of times. 
     
     
       18. An engine system, comprising:
 an engine including a plurality of cylinders; 
 a fuel injection system including a low pressure lift pump, a port injection fuel rail coupled to the lift pump via a fuel line, a plurality of port injectors coupled to the plurality of cylinders, and a pressure relief valve coupled to the fuel line, upstream of the fuel rail; 
 a pressure sensor and a temperature sensor coupled to the fuel rail; 
 a pedal position sensor for receiving an operator torque demand; and 
 a controller with computer readable instructions stored on non-transitory memory the controller executing the instructions to:
 operate the lift pump until a fuel rail pressure exceeds a threshold pressure, and then disabling the lift pump; 
 sequentially operate each of the plurality of port injectors for a predefined number of injection events; 
 the sequential operating including commanding an injector pulse-width based on the operator torque demand, maintaining a base injection pressure and sweeping injection voltage, and maintaining a base injection voltage and sweeping injection pressure; 
 for each of the plurality of port injectors, update a map of injected fuel mass relative to injector pulse-width by correlating a fuel rail pressure drop at each of the predefined number of injection events to one or more of a slope and an offset of the map, the fuel rail pressure drop correlated as a function of each of injection voltage and injection pressure; and 
 after the predefined number of injection events, operate the plurality of port injectors in accordance with the updated map. 
 
 
     
     
       19. The system of  claim 18 , wherein the controller with computer readable instructions stored on non-transitory memory further includes instructions to:
 estimate an injector current based on each of the injection voltage and a sensed fuel rail temperature; 
 translate the correlated fuel rail pressure drop as a function of the injector voltage to a function of the injector current; and 
 further update the map of injected fuel mass relative to the injector pulse-width based on the injector current; and 
 operate the plurality of port injectors in accordance with the further updated map. 
 
     
     
       20. The system of  claim 19 , wherein the operating the lift pump is performed after a cylinder head temperature is above a threshold temperature.